The present invention relates generally to the deposition of coatings upon substrates by utilizing non-local thermal equilibrium arc plasmas. More particularly, the present invention pertains to a method for the deposition of coatings upon substrates, preferably at atmospheric pressure, by utilizing a non-local thermal equilibrium rotating arc plasma. Substrates which may be coated include conductive or non-conductive substrates for semiconductor applications, and glass microballoons utilized as nuclear fuel targets for inertial confinement fusion processes.
Generally, plasma methods for coating substrates such as semiconductor substrates and laser fusion targets are low pressure processes. One method used in the coating of fusion targets is magnetron sputtering either in a batch of bouncing targets or with one target levitated on a gas jet. However, if the targets are bounced during the deposition process, the coating can be damaged or the targets tend to adhere to the bouncing plate or to each other due to either electrostatic interactions or cold welding. Levitation processes using gas jets during the coating process are hampered due to the electrostatic interaction between the target and the levitator, while the low pressure required for sputtering hampers the levitation process.
The deposition of semiconductor materials upon semiconductor substrates has been effected using low pressure deposition processes such as thermal or plasma assisted chemical vapor deposition or glow discharge, sputtering, and thermal evaporation. The low pressure plasma deposition processes characteristically exhibit low deposition rates, and where plasma assist is used, low yields of plasma chemical reactions. It is desired, however, to maintain the non-local thermal equilibrium characteristics of the low pressure plasma, which include the existence of free atoms and radicals existing at low temperatures. In non-local thermal equilibrium plasmas, there exist low densities of electrons and molecular species. Additionally, the temperature of the atomic, free radical and molecular species, in ground, ionized or excited states, is much less than the temperature of the electrons.
The use of high pressure plasmas for the deposition of coating materials upon substrates has been proposed, as in U.S. Pat. Nos. 4,003,770 and 4,292,342.
U.S. Pat. No. 4,003,770 describes a normal plasma spraying process, in which the material is heated with the arc such that molten particles of the coating material are ejected onto the substrate. This is not tailored to allow specific desired chemical reactions to take place, as in the present invention where atomic-sized species of the coating material or materials are activated and these activated species are deposited upon the substrate.
U.S. Pat. No. 4,292,342 describes a high pressure plasma process which utilizes a local thermal equilibrium process. Deposition rates using high pressure plasma sources are increased, and the yield of plasma chemical processes is greater. However, when utilizing high pressure local thermal equilibrium plasmas, there are virtually no activated atomic or free radical species at low gas temperatures. All species, including electron, molecular, atomic and radical species exist at nearly the same temperature. Quenching of the deposition reaction is often necessary, and the high temperature involved severely limits the types of deposition materials and chemical interactions which may be utilized.
It is therefore an object of the invention to provide a method for the deposition of coating materials upon substrates utilizing a high pressure plasma source to activate species of the coating material for deposition, while maintaining non-local thermal equilibrium characteristics such that the activated species exist at a temperature below the electron temperature for the plasma.